Artificial salt water solar ponds are presently used as solar collectors in order to provide a source of low grade heat for conversion into electricity. Such ponds have a 3-layer regime: an upper convective wind-mixed layer at the surface with an average salinity of 3-5%, and with a depth of 30-50 cm., depending upon wind conditions; an intermediate, non-convective layer termed a halocline, about 1-1.5 m. deep, with a salinity that increases uniformly with depth from about 5% at the top to about 30% at the bottom; and a lower heat-storage layer, from 3-5 m. deep, depending on the amount of heat storage desired, with a uniform salinity of about 30%.
Solar radiation incident on the surface is absorbed within the layers. Heat absorbed within a stratum of the wind-mixed layer reduces the density of the stratum, and creates buoyant water which quickly reaches the surface, dissipating the absorbed heat into the atmosphere. Thus, the temperature of the wind-mixed layer approximates ambient temperature. However, heat absorbed in the halocline and in the heat-storage layer is trapped in these layers. The halocline is nonconvective because the density based stratum is so large, compared with the stratum immediately above, that any decrease in density is due to a temperature rise in the lower stratum as a consequence of the absorption of solar radiation is insufficient to materially change its buoyancy. As a result, solar radiation establishes a temperature profile that matches the salinity profile, in the pond. The halocline thus serves as a transparent, insulating cover for the heat-storage layer, and protects the latter against conductive heat loss to the atmosphere. From actual experience with solar ponds, the halocline is remarkably stable for long periods of time, because the rate of salt diffusion is so slow. However, the halocline is particularly sensitive to the effects of wind at the surface. Gusts of wind that create surface waves cause mixing at the surface and which increases the depth of the wind-mixed layer at the expense of a halocline. By driving the wind-mixed layer deeper into the halocline, the average density of the water in the wind-mixed layer increases, and it is conventional to attempt to repair this damage to the halocline by flushing the surface with fresh water, and/or carrying out a selective mixing process at the interface between the wind-mixed layer and the halocline.
The primary reason for attempting to minimize the depth of the wind-mixed layer arises because the rate at which heat can be added to the heat-storage layer from solar radiation is reduced as the wind-mixed layer depth increases. It has been found that an increase of one centimeter in the depth of the wind-mixed layer may reduce the thermal head achieved by the solar pond by about 1.degree. C. When the thermal head is in the neighborhood of about 50.degree. or 60.degree. C., this reduction in thermal head is very costly in terms of the operation of the solar pond.
In an effort to control the depth of the mixed-layer, it is conventional to utilize surface windbreaks; and to this end, nets of low hyper-density polypropylene, for example, have been floated on the surface of the pond. These nets are anchored on the banks or rim of the pond and have proven to be effective in reducing the effect of wind mixing. However, a number of problems have developed with this approach. First of all, the cross-pieces of the net must be relatively close to supress wind formation with the result that the shadow cast by the nets may represent as much as 5% of the total area of the pond; and the nets thus reduce the efficiency of the pond as a collector. In addition, the mineral precipitates collect on the exposed surface of the nets, increasing their weight and causing them to sink below the surface where their effectiveness as a wave suppressor is reduced.
Much effort has been expended in an effort to minimize the depth of a wind-mixed layer in a salt water solar pond; but to date the problem still exists. It is therefore an object of the present invention to provide a new and improved method of and apparatus for dynamically stabilizing the wind mixed layer of a salt water solar pond in a way that does not suffer from the deficiencies of the prior art.